The present invention relates to semiconductor magnetoresistive devices, also known in the art as magnetoresistors, employed in position and speed sensors, and more particularly to a malfunction detector which uses the output signal to monitor the functionality of speed and position sensors.
It is well known in the art that the resistance modulation of magnetoresistors can be employed in position and speed sensors with respect to moving ferromagnetic materials or objects (see for example U.S. Pat. Nos. 4,835,467, 4,926,122, and 4,939,456). In such applications, the magnetoresistor (MR) is biased with a magnetic field and electrically excited, typically, with a constant current source or a constant voltage source. A magnetic (i.e., ferromagnetic) object rotating relative, and in close proximity, to the MR, such as a toothed wheel, produces a varying magnetic flux density through the MR, which, in turn, varies the resistance of the MR. The MR will have a higher magnetic flux density and a higher resistance when a tooth of the rotating target wheel is adjacent to the MR than when a slot of the rotating target wheel is adjacent to the MR. Angular position information is contained in the location of target wheel tooth edges (i.e., tooth/slot transitions), and at these locations the output signals of the MRs are by design unequal so that their differential signal is nonzero.
High accuracy and repeatability magnetic position sensors employ two matched sensing elements such as magnetoresistors or Hall generators. They are spaced a few millimeters apart from each other, either in the axial direction (dual track target wheels) or along the target periphery (sequential sensors). The primary purpose of using two matched sensing elements is common mode signal rejection, since the sensing elements are equally affected by temperature and air gap. Presently, selection of matched MR pairs, a tight process control during all phases of sensor manufacture with a final testing of each sensor, is employed to build sensors meeting the required specifications. Unfortunately, this approach increases the final cost of the sensor.
Currently, variable reluctance (VR) sensors are the most common types of anti-lock braking system (ABS) wheel speed sensors. They are rugged and inexpensive, but are incapable of sensing zero wheel speed. A feature demanded by an increasing number of sophisticated ABS implementations. Also, they do not lend themselves to easy monitoring and automated fault detection. In contrast, semiconductor magnetoresistors manufactured from InSb, InAs, and other compound semiconductors provide large signal outputs down to zero wheel speed and, being resistors, they allow for continuous monitoring and fault detection by simple means without interfering with the wheel speed sensing process.
FIG. 1 shows a present wheel speed sensor utilizing a single MR sensor 100 driven by a constant current source 120 powered by a supply voltage VB with output voltage VS wherein the passage of a tooth 140 of the rotating target wheel 180 produces a high output voltage and the passage of a slot 160 produces a low output voltage. A constant current source 100 is the preferred drive method for single MR sensors. The use of a constant voltage drive, however, would not affect a malfunction detection system.
FIG. 2 depicts the output voltage VS corresponding to the two extreme operating conditions within the specified tolerance range of the sensor 100 as VS1 and VS2. The MR sensor 100 will produce the highest output voltage signal VS=VS1 when the MR sensor is simultaneously operating at the lowest temperature, smallest air gap, and largest MR drive current, all within the specified tolerance range, however. The MR sensor 100 will produce the lowest output voltage signal VS=VS2 when the MR sensor is simultaneously operating at the highest temperature, largest air gap, and smallest MR drive current, all within the specified tolerance range, however. The voltage span between the largest value of VS=VMAX and the smallest value of VS2=VMIN defines the correct operating range of the sensor 100 with a corresponding output signal voltage VS. That is, VMIN less than VS less than VMAX. Monitoring a failure of the MR sensor 100 requires that the maximum output voltage VS not exceed VMAX and that the minimum output voltage VS will not fall below VMIN.
The output signal VS exceeding VMAX may, for example, be indicative of such potential problems as too large a MR drive current 120, defective MR die, bad wiring, bad connector, insecure sensor, or loose target wheel mount. The output signal VS falling below the value of VMIN may, for example, be indicative of a partial short circuit, total short circuit, insufficient MR drive current 120, defective MR die, insecure sensor, or loose target wheel mount.
FIG. 3 shows a present wheel speed sensor utilizing a dual MR sensor 200 driven by a constant supply voltage Vxe2x80x2B with output voltage Vxe2x80x2S wherein the passage of a tooth 240 of the rotating target wheel 280 produces a high output voltage and the passage of a slot 260 produces a low output voltage. A constant voltage source Vxe2x80x2B is the preferred drive method for dual MR sensors. The use of constant current drives, however, would not affect a malfunction detection system.
FIG. 4 depicts the output voltage Vxe2x80x2S corresponding to the two extreme operating conditions within the specified tolerance range of the sensor 200 as Vxe2x80x2S1 and Vxe2x80x2S2. The MR sensor 200 will produce the highest output voltage signal Vxe2x80x2S=VS1 when the MR sensor is simultaneously operating at the lowest temperature, smallest air gap, and largest MR drive voltage Vxe2x80x2B, all within the specified tolerance range, however. The MR sensor 200 will produce the lowest output voltage signal Vxe2x80x2S=Vxe2x80x2S2 when the MR sensor is simultaneously operating at the highest temperature, largest air gap, and smallest MR drive voltage Vxe2x80x2B, all within the specified tolerance range, however. The voltage span between the largest value of Vxe2x80x2S1=VMAX and the smallest value of Vxe2x80x2S1=Vxe2x80x2MIN defines the correct operating range of the sensor 200 with a corresponding output signal voltage Vxe2x80x2S.
That is, Vxe2x80x2MIN less than Vxe2x80x2S less than Vxe2x80x2MAX. Monitoring a failure of the MR sensor 200 requires that the maximum output voltage Vxe2x80x2S not exceed Vxe2x80x2MAX and that the minimum output voltage Vxe2x80x2S will not fall below Vxe2x80x2MIN.
The output signal Vxe2x80x2S exceeding Vxe2x80x2MAX may, for example, be indicative of such potential problems as too large a MR drive voltage Vxe2x80x2B, one or two defective MR dies, bad wiring, bad connector, insecure sensor, or loose target wheel mount. The output signal Vxe2x80x2S falling below the value of Vxe2x80x2MIN may, for example, be indicative of a partial short circuit, total short circuit, insufficient MR drive voltage Vxe2x80x2B, one or two defective MR dies, insecure sensor, or loose target wheel mount.
Accordingly, it is necessary when monitoring either single or dual MR speed and position sensors for malfunction to observe the value of the output signal within a maximum and minimum voltage envelope.
The present invention is a sensor malfunction detection method and system applicable to MR speed and position sensors. The sensor malfunction detection method and system is used in conjunction with passive MR sensor configurations such as depicted in FIGS. 1 and 2 wherein the sensor contains only the MR or MRs and no processing electronics. The raw MR signal VS or Vxe2x80x2S of FIGS. 1 and 2 could be transmitted to an off-site processor containing the malfunction detection circuitry. If the processing electronics is integrated with the sensor, the malfunction detection circuitry could also be integrated with the sensor. However, some fault detecting functions would be difficult to perform within the sensor such as, for example, an integrity check for the connections between the integrated sensor and the controller receiving the sensor output signal. The present invention would be particularly beneficial in ABS where the absence of signal pulses would normally be interpreted as the wheel not rotating.
The present invention compares a voltage output of a MR sensor to a maximum and minimum reference voltage to determine if a failure has occurred. A malfunction detector for magnetoresistor sensors includes a window discriminator, a rotation indication circuit for providing a position signal used by a controller to determine position and speed, and an alternative failure circuit. The window discriminator monitors whether the sensor output voltage is within the correct range. The rotation indication circuit monitors wheel rotation and the alternative failure circuit monitors the magnitude of the raw MR signal in order to recognize failure modes undetectable by the window discriminator such as overheating of the dual MR sensor which may keep the sensor signal within the correct voltage range when its magnitude might become unacceptably low.
Accordingly, it is an object of the present invention to provide a malfunction detector for MR speed and position sensors which observes values of the output signal within a maximum and minimum voltage envelope to monitor the functionality of speed and position sensors.
This, and additional objects, advantages, features, and benefits of the present invention will become apparent from the following specification.